Linear viscoelasticity and dynamics of suspensions and molten polymers filled with nanoparticles of different aspect ratios

In the present review, we report the linear viscoelasticity of suspensions and polymers filled with nano-size particles of different aspect ratios and structuration. The viscoelastic behaviour of liquid suspension filled with well-dispersed and stabilised particles proves that the Brownian motion is the dominant mechanism of relaxation. Accordingly, dilute and semi-dilute suspensions of stabilised carbon nanotubes, cellulose whisker and PS nanofibres obey a universal diffusion process according to the Doi-Edwards theory. Regarding spherical particles, the Krieger-Dougherty equation is generally successfully used to predict the zero shear viscosity of these suspensions. Regarding fractal fillers, two categories can be considered: nanofillers such as fumed silica and carbon black due to their native structure; and secondly exfoliated fillers such as organoclays, carbon nanotubes, graphite oxide and graphene. The particular rheological behaviour of these suspensions arises from the presence of the network structure (interparticle interaction), which leads to a drastic decrease in the percolation threshold at which the zero shear viscosity diverges to infinity. Fractal exponents are then derived from scaling concepts and related to the structure of the aggregate clusters. In the case of melt-filled polymers, the viscous forces are obviously the dominant ones and the nanofillers are submitted to strong orientation under flow. It is generally observed from linear viscoelastic measurements that the network structure is broken up under flow and rebuilt upon the cessation of flow under static conditions (annealing or rest time experiments). In the case of platelet nanocomposites (organoclays, graphite oxide), a two-step process of recovery is generally reported: disorientation of the fillers followed by re-aggregation. Disorientation can be assumed to be governed by the Brownian motion; however, other mechanisms are responsible for the re-aggregation process. (C) 2013 Elsevier Ltd. All rights reserved.